High resolution lithographic process

ABSTRACT

A lithographic process utilizing soft x-rays or ions to achieve high resolution is disclosed. The process is particularly useful and semiconductor processing where high resolution is required to achieve a high density. The process utilizes a mask to selectively expose a photoresist to soft x-rays or flood beams of ions. The mask comprises a thin metallic foil supported by a frame such that the foil is in tension. The frame includes optical alignment keys. A second patterned layer of metal is affixed to the foil to form areas which are nontransparent to the soft x-rays or flood ion beams for delineating the elements of a semiconductor circuit, for example. This permits the mask to be optically aligned using conventionally techniques with high resolution being achieved do to the short wavelength of the x-ray radiation or the ion beams.

This is a division of application Ser. No. 459,286, filed Jan. 19, 1983,now U.S. Pat. No. 4,454,209.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to semiconductor processing and more specificallyto high resolution lithographic processes using soft x-rays or ionbeams.

2. Description of the Prior Art

One of the fundamental limitations of high density in semiconductorcircuits has been the resolution of the litographic processes.Techniques currently offering the most promise of improving theresolution of lithography is either the so called electron beam writing,processes using flood beams of short wavelength radiation such as x-raysor flood beams of ions. Electron beam writing while providing highresolution is a relatively low production technique due to the timerequired for the beam writing process. X-ray and flood ion beamlithography has been difficult because the masks are fragile due to thefact that the preferred mask is a thin metal foil much as beryllium. Inaddition to the fragile nature of such mask, alignment has beendifficult due to the fact that a preferred alignment technique is anoptical process (i.e., using visible light) and the metallic foil maskis non-transparent in the visible range.

SUMMARY OF THE INVENTION

The invention provides a high resolution lithographic process whichutilizes a mask which can be aligned using conventional opticaltechniques. The mask and short wavelength radiation, such as soft x-raysor ion beams, are used to selectively expose resist layers to formpatterned protective layers.

The mask includes a support frame which is transparent to visible lightwith patterns affixed thereto to form optical alignment keys. A foil,preferably of metal, and transparent to soft x-rays or ions is affixedto the lead frame and held in tension thereby. A patterned layer,preferably of a second metal, is affixed to the foil to render selectedareas of the foil nontransparent to soft x-rays or ions. In practicingthe disclosed lithographic process, the mask is utilizied to selectivelyexpose resist layers whose chemical structure can be changed in responseto exposure to soft x-rays or to ion beams. Following exposure, theresist layers are developed to produce a patterned protective layer tobe used in the lithographic process.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section of the support mask frame and the foil bondedthereto.

FIG. 2 is a drawing illustrating how the flat surface comprisingportions of the support frame and the foil is formed.

FIG. 3 is a drawing illustrating the process for bonding the foil to thesupport frame.

FIG. 4 is a cross section drawing of the support frame and the foilbonded thereto with thin overlying layers of a metal and photoresist.

FIG. 5 is a drawing illustrating the electron beam write-in processutilized to form the mask.

FIG. 6 is a cross section view of the final mask.

DETAIL DESCRIPTION

FIG. 1 is a cross sectional drawing illustrating the support frame 10and the thin metallic foil 12, preferably beryllium having a thicknessin the order of 25 microns, bonded thereto. In the preferred embodiment,the outer and inner perimeters, 11 and 13, of the support frame may becircular, rectangular or any other convenient shape. Since the supportframe 10 in glass, it is not normally transparent to soft x-rays. Thislimits the useful area for forming masking patterns useful in soft x-raylithographic techniques to the window formed by the inner perimeter 13of the support frame 10. Areas of the support frame 10 extending beyondthe outer perimeter 15 of the foil 12 are utilized to form keys foraligning the mask using visible light.

A substantially flat common under surface 14 is formed by the surface ofthe foil 12 and the support frame 10. The foil 12 is bonded to thesupport frame 10 under conditions which assure that the foil 12 willalways be under tension in all directions thus assuring a stable flatsurface for the foil 12.

FIG. 2 illustrates the first step in the process for bonding the foil 12to the support frame 10. A first substantially flat plate 16, ofstainless steel for example, is used to support the foil 12. The glasssupport frame 10 is positioned such that the inner edge 13 of thesupport frame 10 overlaps the outer edge 15 of the foil 12. A secondflat plate 18 is positioned in overlying relationship with the glasssupport frame 10. Pressure is applied to the two plates 16 and 18 andthe whole structure is heated to the softening point of the glasssupport frame 10. Under these conditions the glass support frame 10 willbe deformed such that the bottom edges of the glass support frame 10become substantially flat with the bottom surface of the foil 12. Thisproduces the substantially flat bottom surface as illustrated in FIG. 1and previously discussed.

FIG. 3 functionally illustrates the process by which the foil 12 isbonded to the glass member 10. Basically, the foil 12 is electricallyconductive because it is preferably a metal such as beryllium. Heatingthe glass frame 10 to the point where it becomes plastic also causesthis member to become slightly electrically conductive. A voltage source17 of sufficient magnitude is coupled to apply a voltage between thefoil 12 and the frame member 10 to cause an electric current to flowbetween these two members. Bonding time is a function of temperature andcurrent flow. This bonding process is known in the art and fullydiscussed in in U.S. Pat. No. 3,397,278 issued to D. I. Pomorantz. Forpurposes of illustrating how the glass to metal bond is made, U.S. Pat.No. 3,397,278 is incorporated by reference.

The bonded operation described above will take place normally in atemperature range in from 300° to 800° C. depending upon the glassselected for the support frame 10. Also the glass member 10 willgenerally have a slightly lower coefficient of expansion than themetallic foil 12. When the bonded operation has been complete and theframe member 10 and the foil 12 are cooled the foil 12 will be undertension assuring that the common surface 14 of the class support frame14 and the foil 12 will be substantially flat, as discussed above andillustrated in FIG. 1.

After the support frame 10 and the foil 12 have cooled, the bottomsurface including the support frame 10 and the foil 12 are coated with athin layer of gold 20, preferably having a thickness in the range of 1/2to 1 micron. To facilitate adhesion of the gold layer 20 the bottomsurface of the glass support frame 10 and the beryllium layer 12 may befirst covered with a very very thin layer of titanium for example. Onthe surface of the gold layer 20 a thin layer of photoresist 22 isdeposited.

Electron beam apparatus functionally illustrated at reference numeral 24is utilized to selectively expose the photoresist 22. The pattern to beutilized for X-ray photolithographic purposes is confined to an areawithin the window formed by the inner perimeter 13 of the support frame10 because the glass support frame will not normally be transparent tothe soft x-rays. The regions overlying the edges of the support frame 10which extend beyond the edge of the foil 12 are utilized to form opticalalignment keys which are utilized to align the mask using standardtechniques and visible light. The electron beam write-in apparatus 24can be controlled in a conventional manner to produce the desiredpattern for the lithographic processes as well as the alignment keys.

Following electron beam writing, the photoresist layer 22 is developedusing conventional processes and the gold layer 20 is etched to producea pattern for lithographic processes within the window defined by theinner perimeter 13 of the support frame 10 and optical alignment keys inthe portions extending beyond the outer perimeter 15 of the foil 12. Thefinish mask is illustrated in FIG. 6.

The gold images can also be formed using rejection or plating processes.In all cases, the technology for forming the gold images is well knownin the art.

The mask illustrated in FIG. 6 is particularly useful in x-ray or floodbeam ion lithographic process in of semiconductor wafers. A typicalphotolithographic process is functionally illustrated in FIG. 7. Asemiconductor substrate is coated with a thin layer of photoresist 28using conventional process in techniques. The mask is the positionedsuch that the gold pattern layer 20 is in close proximity or actuallytouching the photoresist layer 28. The alignment keys and the edges ofthe mask utilized to properly align the mask with respect to thesubstrate 26 using well-known optical alignment techniques. Followingalignment, a flood beam of soft x-rays or ions is directed to impinge onthe upper surface of the mask. The soft x-rays or ions will notpenetrate the glass forming in the support frame 10 or the portions ofthe metallic foil 12 which are overlaid by the patterned gold layer 20.However, in regions where there are openings in the gold patterned layer20 which are within the inner perimeter 13 of the support frame 10, thesoft x-rays or ions will penetrate the foil 12 and impinge upon thephotoresist layer 28. The chemical structure of the photoresist layer 28is modified by the x-rays or the ions. The photoresist layer 28 is thendeveloped and processed to use in conventional processing.

In the above-described process, soft x-rays and ions are utilizedbecause currently available resists are not suitable for use with hardx-rays. It should also be noted that in photolithographic processing ofsemiconductor wafers that alignment is only critical during the secondand subsequent photolithographic processing steps. Thus, the first maskshould be designed such that the inner perimeter 13 of the support frame10 is larger than the active area of the circuit. During the firstprocessing step, alignment patterns must be produced for aligning withthe alignment keys. Thus, it is obvious that the mask described abovt isthe type for use with the second and subsequent processing steps becauseit includes alignment keys in the portions of the support frame 10 whichextend beyond the perimeter 15 of the foil 12.

What we claim is:
 1. A method for photolithographic processing ofsemiconductor wafer comprising the steps of:(a) positioning a first maskhaving a metallic central portion and a glass edge portion in overlyingrelationship with a semiconductor wafer covered with a resist layer; (b)directing a flood beam of radiation of a first wavelength shorter thanvisible light to impinge on said mask to selectively expose said resistin an area underlying said metallic portion; (c) developing said resistto form a first portection layer to delineate first areas of the surfaceto be exposed during the first processing steps of a semiconductorcircuit and areas to form optical alignment keys to be used to alignsubsequent mask for subsequent photolithographic processing steps; (d)processing said semiconductor wafer to modify said first areas inaccordance with the processing requirements determined by thespecifications of the device being manufactured and said second areas toform keys suitable for optical alignment of mask used in subsequentprocessing steps; (e) coating said semiconductor wafer with a secondlayer of resist; (f) positioning a second mask in overlying relationshipwith said second resist layer, said second mask having:(1) a glasssupport frame transparent to visible light and non-transparent to shortwavelength radiation, (2) a beryllium foil non-transparent to visiblelight and transparent to said short wavelength radiation, said foilbeing affixed to said support frame such that said support frame extendsbeyond the perimeter of said foil with said foil and said portions ofsaid support frame which extend beyond the perimeter of said foiljoining to form a substantially flat surface, and (3) a thin patternedlayer affixed to said substantially flat surface rendering portions ofsaid beryllium foil and said support frame overlaid by said patternedlayer non-transparent to visible light and to radiation within saidselected band with portions of said support frame in conjuction withportions of said patterned layer forming a key for aligning said maskpermitting said mask to be aligned using visible light; (g) Opticallyaligning said second mask with said keys using radiation within thevisible spectrum and corresponding keys in said glass portion of saidsecond mask; (h) directing a flood beam of radiation of said firstwavelength to impinge on said second mask to selectively expose saidresist layer; (i) developing said resist to form protective layersdelineating areas of said semiconductor wafer to be modified during thenext processing steps; (j) processing the semiconductor wafer to modifythe delineated areas.